Polymer-based film, preparation method therefor, and use thereof

The present invention relates to the technical field of a membrane, a preparation method therefor and use thereof, particularly, to a polymer-based membrane with a loofah sponge-like structure, a preparation method therefor and use thereof, and a functional material comprising the membrane, in particular a filter membrane or a separation membrane.

Nanofibers in the nature, such as spider silk and silkworm silk, have received extensive attention for their excellent properties, but their industrialization has been limited due to the lack of natural resources. In order to realize the artificial preparation of nanofibers, researchers have been making explorations for many years. Among them, electrospinning technology has become one of main routes for effective preparation of nanofiber materials due to its advantages of simple manufacturing equipment, low spinning cost, a wide variety of spinnable substances, and controllable process.

Electrospinning is a process wherein under a strong electric field, the droplets formed by a polymer solution or melt at the needle tip will change from spherical to conical shape, and extend from the conical tip to obtain the fiber filaments, to thereby perform jet spinning, and at a receiving device, the spun filaments are solidified. In this way, polymer filaments of several nanometers to several micrometers in diameter can be produced. In recent years, due to their high specific surface area, high porosity and special physical and chemical properties, electrospun fibers have been widely used in environmental protection, health, energy and other fields, such as high-efficiency filter and separation membrane materials in environmental governance, membrane materials for energy storage and conversion in energy devices, tissue culture and wound dressing materials in the medical field, etc. Researchers endow nanofibers with different morphologies and functions mainly through the means such as material modification (CN109713203A), composite of multiple materials (J. Power Sources, 2014, 261, pages 1-6), and morphology control (Adv. Funct. Mater. 2018, 28, 1705051).

However, the preparation of nanofiber membranes by electrospinning technology still faces some problems to be solved. Although electrospinning devices of various industrial scales with different types of spinning/collecting accessories have been devised, the yields are often too low. In the current state of the art, the spinning efficiency is low, with a maximum of several grams per hour for each needle, and the yield of one device limited to dozens of kilograms per day, so that the application of final products is mostly in the laboratory stage only. In addition, high-voltage electricity brings operational risks to workers; and in the process of solution electrospinning, the solvent usually accounts for 70-90 wt % of the solution, and the evaporation of the solvent into the environment will bring about environmental burden and safety problems, and waste the chemicals; when a flammable organic solvent is used, the organic solvent readily volatizes and will generate a large amount of flammable gas, resulting in fire hazards and environmental risks. Therefore, it is particularly important to design a high-yield spun nanofiber separation membrane using an environmentally friendly technology. In addition, the nanofiber membrane prepared by the electrospinning method is present in the form of non-woven fabric, with its microstructure shown in the electron microscope photograph in(from J. Mater. Chem. B 2014, 2, 181-190), the fibers inside the membrane are generally in a state of overlapping with each other in layers, and the fibers of the various layers are not firmly fixed together. When it is applied to occasions in need of withstanding large impact forces such as liquid filtration, its structural stability is insufficient.

On the other hand, the demand for water resources in modern society is increasing, but the population and economic development ever since the industrial revolution has inevitably produced a large amount of waste water, especially oily waste water, which needs to be properly treated for recycling. Membrane water treatment technology is widely used in water treatment process due to its simple process, low cost, energy saying and high efficiency, especially for the separation of oil-water mixture.

The most difficult to separate in oily wastewater is the emulsified oil. The oil droplets have particle diameter mainly between 0.1 and 2 μm, usually less than 10 μm, and are stably dispersed in water in the oil-in-water form. For the treatment of such oily sewage, the study on the superhydrophilic and superoleophobic separation membrane materials has received extensive attention.

Microfiltration membranes occupy a place in the field of membrane separation due to their small pore size and relatively low cost of use. Microfiltration membranes mainly rely on mechanical sieving to allow macromolecules and soluble solids to penetrate, but intercept the substances such as larger-size suspended matters, bacteria, and high-molecular-weight colloids.

For liquid separation membranes, mostly, there is the irreversible pollution problem during the separation process. Therefore, it is necessary to develop a separation membrane with a suitable surface microstructure while ensuring permeability, so as to reduce the adhesion of pollutants to the membrane surface to thereby improve its service life. For example, in the oil-water separation process, usually the solid-oil-water three phases contact, and in order that the oil droplets do not stick on the surface and readily roll, it is ideal to build a rough micro-nano structure surface with discontinuous three-phase contact lines. The current reports on the construction of micro-nano structure separation membranes include the preparation of the salt-induced phase separation PAA-g-PVDF membrane (Angew. Chem. Int. Ed. 2014, 53, 856-860), which is used for the effective separation of oil-water emulsions, but has high raw material costs and brings a large amount of high-salinity wastewater. In addition, for example, CN109316981A reported that hyperbranched polyether was grafted to the surface of a hydrophilically modified membrane, and its numerous long branches could go deep into the oil-water interface of the emulsion to destroy the strength of the emulsion interface membrane to thereby achieve the effect of demulsification separation. In CN109046034A, polyvinylidene fluoride was modified by polydopamine/silica powder to obtain a hydrophilic/oleophobic vinylidene fluoride separation membrane. The literature Adv. Funct. Mater. 2018, 28, 1705051 reported that the surface of lotus leaf-like structure was constructed by the combination of electrospinning and electrostatic spraying, the prepared separation membrane had a high porosity, the mastoid structures similar to lotus leaf surface were distributed on the membrane surface, and the membrane exhibited high separation efficiency for 0/W (oil-in-water) emulsions while resisting contamination. In addition, there are organic-inorganic hybrid metal mesh membrane (CN110280222A), chemical grafting on membrane surface (CN109499393A) and other methods.

However, most of the existing methods for preparing separation membranes with a micro-nano composite structure have the disadvantages that the material cost and process cost are high, process operation is cumbersome, continuous preparation cannot be achieved, industrialization is difficult, and the micro-nano rough structure on the membrane surface cannot be maintained for a long time, or the separation application range of the membrane is narrow.

In addition, in the conventional vapor-induced phase separation (VIPS) method for membrane preparation, the non-solvent is introduced into the polymer solution from the gas phase. In this case, the precipitation is very slow. Due to the slow introduction of the non-solvent, there is no membrane-forming liquid concentration gradient in the membrane thickness direction. The membrane-forming liquid precipitates almost simultaneously along the thickness direction of the entire film. However, since the membrane-making process of the VIPS method is very slow, the membrane-making time is usually several hours, and the efficiency is low, it is difficult to achieve industrial continuous production and not suitable for the preparation of practical separation membranes or filter membranes, especially porous membranes cannot be prepared when porous materials such as non-woven fabrics are used as the support layer.

Therefore, there is a need for an improved membrane suitable for separation or filtration, which has an improved overall performance and can be prepared efficiently with raw materials of lower price, at low cost, and by a simple process.

In view of the above problems in the prior art, the object of the present invention is to provide a membrane with a highly penetrating-through pore (hole) structure and a three-dimensional fiber network structure, which has a good separation or filtration effect, is particularly suitable for use as a microfiltration membrane for oil-water separation and has a high structural stability.

In particular, it is also an object of the present invention to provide a membrane having a special surface microstructure, which has a special wettability, thereby improving the fouling resistance of the membrane.

Another object of the present invention is to provide a method for preparing the membrane, which method is efficient, low in cost, and simple in preparation process, and wherein the membrane can be prepared based on general-purpose polymers, thus the raw material price is low.

According to the present invention, it has been unexpectedly found that by the method of combining atomization pretreatment with non-solvent induced phase separation (NIPS, also known as non-solvent phase inversion), an organic polymer-based membrane having a loofah sponge-like structure is prepared with high efficiency, the loofah sponge-like structure comprises a fiber skeletal structure formed by three-dimensionally interwoven and interconnected polymer fibers, and a three-dimensionally interpenetrating network pore structure distributed in the fiber skeletal structure, and particularly in the case of using at least two polymers, a micro-nano composite structure with nano-scale protrusions on the fiber skeleton is obtained, thereby achieving the objects.

The pores of the surface of the membrane according to the present invention and of the penetrating open-pore structure in the internal structure of the fiber skeleton constitute the gaps similar to those among the overlapping fibers in the nanofiber membrane structure prepared by electrospinning, which can achieve a similar or better effect in separation, filtration, adsorption and the like. Nevertheless, the membrane of the present invention differs from the nanofiber membrane obtained by electrospinning in that the polymer of the membrane of the present invention shows a three-dimensional fiber network structure, and the polymer fibers are directly three-dimensionally interconnected, i.e., show radial connections in three-dimensional space, rather than overlapping connections. The connections among the polymer fibers of the membrane of the present invention are firm, thereby improving the structural stability and strength of the membrane.

Membrane

Thus according to a first aspect, the present invention provides a polymer-based membrane having a loofah-sponge like structure comprising a fiber skeletal structure formed by three-dimensionally interwoven and interconnected polymer fibers and a three-dimensionally interpenetrating network pore structure distributed in the fiber skeletal structure, wherein the polymer is an organic polymer and the fiber skeletal structure is integrally formed from the polymer, and the volume porosity of the membrane is 50%-95%. This is a membrane having a highly penetrating-through bicontinuous network pore structure.

The “membrane” mentioned herein refers to a functional material having a film-like structure, such as a separation membrane, a filter membrane, and the like.

As used herein, “loofah sponge-like structure” refers to a structure similar to a loofah sponge structure, which comprises a fiber skeletal structure formed by three-dimensionally interwoven and interconnected polymer fibers, and a three-dimensionally interpenetrating network pore structure distributed in the fiber skeletal structure, as is shown, for example, in. The loofah sponge structure is shown in, which is a photograph of the loofah sponge. It can be seen that the internal fiber layer of the loofah sponge structure is of a three-dimensional stereo-structure rather than a laminated structure, that is, the connection points between the same fiber layer and various longitudinal fibers are not in the same plane. The polymer fibers of the membrane of the present invention are three-dimensionally interconnected to form a firmly interconnected three-dimensional network structure similar to the loofah sponge structure shown in, i.e., the loofah sponge-like structure as defined above, in which structure the internal fiber layer is also of a three-dimensional stereo-structure, instead of a laminated structure, that is, connection points between the same fiber layer and various longitudinal fibers are not in the same plane, wherein the pores among the fibers constitute the through-pore structure of the polymer membrane described above. The membrane having such a loofah sponge-like structure according to the present invention has a highly penetrating-through network pore structure, high porosity and high specific surface area, as well as a good structural stability.

As used herein, “three-dimensionally interwoven” means that polymer fibers are distributed in a staggered manner inside and between the fiber layers at different thicknesses, and are not parallel to each other; and the connecting fibers (also referred to as longitudinal fibers) between the various fiber layers are not parallel to each other, the connection points between the same fiber layer and the various longitudinal fibers are not in the same plane, so that the fiber skeletal structure has an irregular shape, and a network pore structure is formed among the fibers, wherein the connection points of the fibers constituting each pore are not in the same plane or inside the same fiber layer.

As used herein, “three-dimensionally interconnected” means that the polymer fibers are interconnected in the surface direction of the membrane, and are connected up and down from the surface of the membrane to the interior thereof, in other words, the polymer fibers are connected radially in three-dimensional space.

As used herein, “three-dimensionally interpenetrating” means that the pore structure of the membrane penetrates up and down from the surface of the membrane to the interior thereof, and penetrates in plane in a direction parallel to the surface of the membrane inside the membrane.

In the membrane of the present invention, the fiber skeletal structure is integrally formed from the polymer. The polymer fibers are directly connected as a whole, and such a membrane is formed from the polymer at one time during the manufacturing process.

In the fiber skeletal structure of the membrane according to the present invention, the average distance between two adjacent connection points in the thickness direction may be smaller than the average distance between two adjacent connection points in the surface direction.

In the cross-section of the membrane according to the invention, substantially the same type of pores can be distributed along the thickness direction of the membrane. As is well known in the art, the types of pores of membrane mainly include network pores, sponge-like pores, finger-like pores and the like. Different types of pore structures often appear simultaneously on the cross-section of the microfiltration membrane or ultrafiltration membrane obtained by the traditional non-solvent induced phase separation method, for example, there is the sponge-like pore structure close to the surface layer of the membrane, and there is large finger-like pore structure close to the middle part and bottom part of the membrane. The cross-section of the membrane of the present invention has network pores distributed along the thickness direction of the membrane and has substantially no other types of pores, such as sponge-like pores and/or finger-like pores. The membrane of the present invention preferably has only network pores distributed from the surface to the interior. The cross-section of the membrane of the present invention can be a structure formed by the polymer fiber skeletal structure and pores with a substantially consistent morphology along the thickness direction of the membrane, that is, on the cross-section of the membrane, the polymer fiber skeletal structure and substantially the same type of pores can be distributed along the thickness direction of the membrane, in other words, the pores distributed on the entire cross-section of the membrane are substantially of the same type, and there is no such phenomenon that the membrane surface layer and the membrane bottom layer have different types of pores. “Substantially the same type” means that there may be inevitably a small number of different types of or imperfect pore structures in the cross-section of the membrane due to the process, but the cross-section of the membrane as a whole is distributed with the same type of pores, i.e., network pores.

Herein, the microfiltration membrane generally refers to a filter membrane with an average pore size of 0.1-10 μm, and the ultrafiltration membrane generally refers to a filter membrane with an average pore size of 10-100 nm.

The cross-sectional diameter of a single polymer fiber between two connection points in the fiber skeletal structure of the membrane according to the present invention may be less than or equal to 2 μm, and may vary irregularly. The diameter of the cross-section of the fiber is determined by scanning electron microscopy (SEM).

As used herein, the “single polymer fiber” refers to a single fiber-like part between two connection points in the fiber skeletal structure. The so-called fiber-like part refers to the part distributed in the form of a thread in the membrane skeleton.

The length of a single polymer fiber between two connection points in the fiber skeletal structure of the membrane according to the present invention can be less than 10 μm, as measured by SEM, and characterized by the linear distance between the starting point and the ending point of the fiber-like part between two connection points in the fiber skeletal structure in the SEM photograph indicated in. The length of such a single polymer fiber is significantly shorter than the length (at least on the centimeter scale) of a single polymer fiber in a nanofiber membrane prepared by the electrospinning method, thereby resulting in a fiber skeletal structure according to the present invention that is significantly different from the fiber structure obtained by the electrospinning method.

The membrane according to the present invention may have an average pore size of from 0.01 to 5 μm, preferably from 0.1 to 3 μm, more preferably from 10 nm to 3 μm. The average pore size of the membrane is measured by a gas permeation method, for example, using a pore size analyzer.

The membrane of the present invention has a highly penetrating-through network pore structure and has high porosity. The volume porosity of the membrane of the present invention can reach 50% to 95%, preferably 65% to 95%, more preferably 80% to 95%. The volume porosity of the membrane is determined by a gravimetric method.

In a preferred embodiment, the polymer fibers of the membrane of the present invention may have a cavity structure inside them, thereby further increasing the porosity, specific surface area and improving adsorption property.

The membrane according to the present invention is based on a polymer and is mainly made of a polymer. The polymer is an organic polymer, including but not limited to any general-purpose polymers suitable for membrane preparation or their modified polymers, preferably can be selected from any polymers suitable for membrane formation by non-solvent induced phase separation method. The polymer may for example be at least one selected from the group consisting of polyvinylidene fluoride, polysulfone, polyethersulfone, polyacrylonitrile, polyacrylic acid, polylactic acid, polyamide, chitosan, polyimide, modified cellulose (e.g., cellulose acetate), polystyrene, polyolefin, polyester, polychlorotrifluoroethylene, polyvinyl chloride, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, modified starch, polyvinylamine hydrochloride, polyethyleneimine, poly-N-isopropylacrylamide and their modified polymers (e.g., polyvinylidene fluoride modified by acrylic acid grafting, sulfonated polysulfone, maleic anhydride-grafted polysulfone, sulfonated polyethersulfone, and acrylic acid-grafted polyacrylonitrile).

The polymers can be hydrophilic polymers or lipophilic polymers so as to prepare membranes with different wettability, such as hydrophilic membranes or hydrophobic membranes.

The hydrophilic polymer can be selected from polymers which per se have a certain degree of hydrophilicity, including but not limited to one or more of sulfonated polyethersulfone, polylactic acid, polyester, chitin, cross-linked polyvinyl alcohol, modified cellulose, modified starch, polyethylene glycol, chitosan, polyacrylonitrile, polyvinylamine hydrochloride, polyacrylic acid, polymer hydrogel (such as poly-N-isopropylacrylamide hydrogel), cellulose acetate, polyethyleneimine, polyamide, polyimide and the like; or can be a polymer modified by hydrophilic group grafting or a block copolymer containing a hydrophilic group, for example, including one or more of copolymers or branched polymers obtained by graft modification with hydrophilic segments such as acrylic acid, maleic anhydride, polyethylene glycol and sulfonic acid group, or obtained by block copolymerization thereof, for example, polyvinylidene fluoride modified by acrylic acid grafting, sulfonated polysulfone, sulfonated polyether sulfone, maleic anhydride-grafted polysulfone, acrylic acid-grafted polyacrylonitrile and the like. Preferably, it can be at least one selected from the group consisting of sulfonated polyethersulfone, polyacrylonitrile, polyacrylic acid, polylactic acid, polyamide, chitosan, polyimide, polyester, chitin, cellulose acetate and the like.

The hydrophobic polymer may be at least one selected from the group consisting of polyvinylidene fluoride, polysulfone, polyethersulfone, polyolefin, polychlorotrifluoroethylene, polyvinyl chloride, polystyrene, and the like.

The molecular weight of the polymer is not particularly limited, as long as it is suitable for membrane formation, particularly, by non-solvent induced phase separation method, to form a three-dimensional fiber skeletal structure.

In one embodiment, the surface of the membrane of the present invention may have micro/sub-micron sized recess structures, with loofah sponge-like structures distributed on or around or among the recess structures. The recess structure can increase the contact area between the liquid and the loofah sponge-like structure or the network pore structure, and can further increase the roughness of the membrane surface, thereby further improving the separation or filtration efficiency of the membrane.

The recess structure may have a size of 0.5 to 10 μm, as determined by SEM, and characterized by the diameter of the opening of the recess on the membrane surface in the SEM photograph. The size of the recess is significantly larger than the size of the pores in the loofah sponge-like structure or the average pore size of the membrane. In this embodiment, the average pore size of the membrane is preferably 10 nm to 3 μm. Such a membrane with recess structures is preferably made of a hydrophilic polymer as the matrix.

The membrane according to the present invention may additionally comprise additives commonly used in membrane preparation, for example, various inorganic nanoparticles, such as nanoscale inorganic fillers, such as MnO, SiO, ZnO, and the like. It may also comprise inorganic salt porogens remaining in the membrane preparation process, such as LiCl, ZnCl, MgCl, LiBr, etc.

The membrane may also be present on a support layer, e.g., on a fabric, preferably a nonwoven fabric.

The membrane of the present invention has a microstructure similar to loofah sponge, wherein the densely distributed three-dimensional network through-pore structure significantly increases the surface roughness (Ra) of the porous membrane, wherein Ra can reach 1 to 10 μm, as determined by non-contact optical profile analysis method. The increase in surface roughness can improve the wettability of the membrane surface, making the hydrophilic surface more hydrophilic and the hydrophobic surface more hydrophobic; and the increase in wettability is beneficial to improve the selective separation function of the membrane. Based on the synergistic effect of the pore structure and surface/interface wettability, the obtained hydrophilic membrane exhibits a strong hydrophilicity and underwater oleophobicity, and has an excellent oil-water separation efficiency, wherein after the membrane surface is in contact with water, a highly stable hydration protective layer can be formed on the surface of the membrane, so as to have the effect of inhibiting the adherence of oil droplets under water, and this effect can be further enhanced when the surface of the membrane has a recess structure; in addition, the obtained hydrophobic membrane exhibits a strong hydrophobicity and lipophilicity. This special wettability allows the membrane according to the present invention to be effectively used in the fields of separation, filtration, adsorption and the like. The membrane according to the present invention exhibits excellent properties such as high flux, high retention rate, self-cleaning, low adherence, high adsorption rate, etc., for example, when used as a separation membrane, for an O/W type emulsified oil having an average oil droplet size of 300 nm to 3 μm, the oil-water separation efficiency can reach above 99%. In addition, since the polymer fibers are three-dimensionally connected to form a firm fiber skeletal structure, the structure and properties of the membrane of the present invention are stable, and the phenomenon of fiber slippage during the use of the electrospinning membrane will not occur.

Membrane with Micro-Nano Composite Network Structure

In one embodiment, nano-scale protrusions can be distributed on the fiber skeletal structure of the membrane, thereby forming a micro-nano composite network structure, which thus realizes special wettability, thereby improving the fouling resistance of the membrane.

The protrusions are integrally formed with the fiber skeleton. The size of the protrusions may be in the range from 20 to 400 nm, as determined by SEM. The protrusions are generally in the shape of particles, and accordingly, the size of the protrusions refers to the average particle diameter of the particles.

In such an embodiment, the average pore size of the membrane may be in the range from 0.1 to 5 μm.

The micro-nano composite network structure or micro-nano structure according to the present invention refers to a structure comprising a micro-scale network skeleton and nano-scale protruding small particles on the skeleton. The presence of such a micro-nano structure enables the membrane to have a special wettability of hydrophilicity in air/superoleophobicity under water and have an extremely low adhesion to oil. The sieving channels formed during the construction of the rough surface of the membrane also endow the membrane with oil-water separation property. The micro-nano structure on the surface enables the surface of the membrane to form a highly stable hydration protective layer after contact with water, which layer blocks the contact between oil droplets and the membrane, thereby achieving the effect of inhibiting the adherence of oil droplets under water. In addition, the network skeleton of the membrane is of a micro-nano composite structure, so that the solid-water-oil three-phase contact line is discontinuous in water, thus oil will not adhere to the membrane while the membrane is superoleophobic in water.

In such an embodiment, the membrane typically comprises a mixture of at least two polymers. Useful polymer types are described above. Preferably, the at least two polymers are hydrophilic polymers that are soluble or miscible in the same good solvent.

Preferably, the first polymer may include, but is not limited to: at least one of polyvinyl chloride, polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride, polyimide, acrylonitrile-styrene copolymers (AS resin) and their modified polymers. The second polymer can be dissolved in a good solvent of the first polymer or miscible with the first polymer, and can include but is not limited to: at least one of chitosan, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol and polyoxyethylene polyoxypropylene ether block copolymer.